Evolution of the one-phonon 21,ms+ mixed-symmetry state in N=80 isotones as a local measure for the proton–neutron quadrupole interaction

An inverse kinematics Coulomb excitation experiment was performed to obtain absolute E2 and M1 transition strengths in 134Xe. The measured transition strengths indicate that the 23+ state of 134Xe is the dominant fragment of the one-phonon 21,ms+ mixed-symmetry state. Comparing the energy of the 21,ms+ mixed-symmetry state in 134Xe to that of the 21,ms+ levels in the N=80 isotonic chain indicates that the separation in energy between the fully-symmetric 21+ state and the 21,ms+ level increases as a function of the number of proton pairs outside the Z=50 shell closure. This behavior can be understood as resulting from the mixing of the basic components of a two-fluid quantum system. A phenomenological fit based on this concept was performed. It provides the first experimental estimate of the strength of the proton–neutron quadrupole interaction derived from nuclear collective states with symmetric and antisymmetric nature.